Mechanical mixers for mixing components to homogeneity are well known. Their applications include, but are not limited to, baking, building construction and medicine.
Mixing apparatus for high viscosity mixtures must be adapted to provide sufficient shear force to continue moving against great resistance. In some cases, the resistance increases during mixing because the viscosity of the mixture increases.
One example of a case where the viscosity of the mixture increases during mixing is preparation of a polymer/monomer mixture. When a polymer and monomer are combined, a polymerization reaction begins. The polymerization reaction increases the average polymer chain length in the mixture and/or causes cross-linking between polymer chains. Increased polymer chain length and/or cross linking between polymer chains contribute to increased viscosity
Polymerization mixtures are often employed in formulation of bone cement. One common polymer/monomer pair employed in bone cement formulation is polymethylmethacrylate/methylmethacrylate (PMMA/MMA). Because PMMA/MMA bone cements typically set to a solid form, reaction conditions for the polymerization reaction are generally adjusted so that mixing PMMA and MMA produces a liquid phase which lasts several minutes. This is typically achieved by mixing a monomer liquid including MMA and, optionally DMPT and/or HQ, with a polymer powder including PMMA and, optionally Barium Sulfate and/or BPO and/or styrene. As a result, previously available mixing equipment is constructed for use with a liquid polymerization mixture and is not well suited to mixing of highly viscous cements that have substantially no liquid phase during mixing.
The following references are cited as being generally indicative of mixer types which are currently available for use in preparation of bone cement. The list does not purport to be exhaustive.
U.S. Pat. Nos. 5,302,020; US 2003/0174576; U.S. Pat. No. 6,994,465 and 4,961,647 disclose use of a central mixing element in combination with a planetary mixing element which revolves around the central mixing element. The disclosure of each of these patents is fully incorporated herein by reference.
U.S. Pat. Nos. 5,415,474 and 7,029,163 disclose a transfer mechanism as part of a mixing apparatus. The disclosure of each of these patents is fully incorporated herein by reference.
U.S. Pat. No. 5,549,381 discloses a wiper which removes adhering mixture from a ribbon configuration mixing element as the mixing element is removed from the mixing apparatus. The disclosure of this patent is fully incorporated herein by reference.